Field of the Invention
The invention relates to an optical filter and an associated method for optical filtering. An optical signal traverses n coupling elements and n-i units with a differential group delay alternately in a conductor structure. The optical signal can be divided between a first coupling element and an nth coupling element into a first and a second signal mode.
Chromatic dispersion impairs high-rate optical data transmission. A chromatic dispersion compensator is described in U.S. Pat. No. 5,259,048. The device is an optical filter that can equalize or generate chromatic dispersion. It is constructed from cascaded Mach-Zehnder interferometers. The is also possible to make use, instead of the customary design with two waveguides, of the two modes, termed signal modes below, of a single waveguide. It is disadvantageous in that case that the adjustment of a relatively large number of propagation time differences is required. This can be accomplished in practice only with difficulty. In particular, it would be necessary for the signal whose dispersion is compensated to be precisely analyzed, and this would cause a high outlay.
Electronics Letters 34(1998)25, pages 2421-2422, specifies a similar optical filter for compensating polarization mode dispersion, one of the differences from U.S. Pat. No. 5,259,048 consisting in that the input signal can already be arbitrarily divided into the two signal modes of the, here single, waveguide, and that the output signal can comprise both signal modes of the single waveguide, and also does so as a rule. Here, as well, the adjustment of the many adjustable voltages is a difficult task requiring at least a great deal of time to accomplish.
It is accordingly an object of the invention to provide an optical filter and an optical filtering method which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which specify simple and effective possibilities for adjusting an optical filter, in particular a chromatic dispersion compensator.
With the foregoing and other objects in view there is provided, in accordance with the invention, an optical filter for an optical signal, comprising:
a number n coupling elements and nxe2x88x921 units with a differential group delay disposed in a conductor structure to be alternately traversed by the optical signal, wherein the optical signal can be divided between a first the coupling element and an nth the coupling element into a first signal mode and a second signal mode;
a measuring device disposed in a beam path of the optical signal, the measuring device outputting a power division signal characterizing a division of a power of the optical signal into a first measurement mode and a second measurement mode;
a control element disposed in the beam path of the optical signal upstream of the measuring device, the control element influencing the division of the power of the optical signal into the first and second measurement modes, and the control element having a control input connected to receive a control signal obtained from the power division signal such that a prescribed division of the power of the optical signal into the first and second measurement modes results.
In other words, the inventive solution to the above problems consists in that the power division between two signal modes, for example signal modes in interferometer arms, is monitored within the optical filter, and is shaped at least approximately, by setting a control element, for example a further interferometer, upstream thereof in the propagation direction, with a controllable phase difference, such that the desired filter properties, for example chromatic dispersion, occur. For first-order chromatic dispersion, equal optical powers are set in the two signal modes or interferometer arms, while for second-order chromatic dispersion the power division between the signal modes or interferometer arms is varied periodically along the optical filter.
The division of the powers into the two signal modes can be selected respectively according to a desired characteristic of the optical filter in order to generate or compensate polarization mode dispersion.
In accordance with an added feature of the invention, the control element is a controllable differential phase shifter, and one of the coupling elements, providing the first and second measurement mode, is connected between the differential phase shifter and the measuring device.
In accordance with an additional feature of the invention, the control element is formed by one of the coupling elements configured to be controllable.
Preferably, the first and second measurement modes are first and second signal modes.
In accordance with another feature of the invention, the coupling element has first and second measurement mode outputs carrying the first and second measurement modes, and an output mode output, between the first and second measurement mode outputs, carrying an output mode of the optical signal.
In accordance with a further feature of the invention, the measuring unit includes devices for forming the power division signal proportionally to a linear combination of the powers of the first and second measurement modes. In a preferred embodiment of the invention, the devices form the power division signal proportionally to a difference between the powers of the first and second measurement modes.
In accordance with again an added feature of the invention, one of the coupling units has a coupling control input connected to receive a dispersion compensation signal for controlling a coupler phase angle thereof.
With the above and other objects in view there is also provided, in accordance with the invention, a method for optical filtering, which comprises:
traversing with an optical signal n coupling elements and nxe2x88x921 units having a differential group delay alternately in a conductor structure, and dividing the optical signal between a first coupling element and an nth coupling element into a first signal mode and a second signal mode;
generating a power division signal characterizing a division of a power of the optical signal into a first and second measurement mode;
generating a control signal for controlling a control element influencing the division of the power of the optical signal into the first and second measurement modes, and thereby generating the control signal from the power division signal such that a prescribed division of the power of the optical signal into the first and second measurement modes results.
In accordance with again an additional feature of the invention, a differential phase shift is generated in the control element, and the first and second measurement modes are provided by a coupling element between the control element and the measuring device.
In accordance with again another feature of the invention, the first and second measurement modes are respectively output at a first and second measurement mode output of the coupling element, and an output mode of the optical signal is provided at an output mode output between the first and second measurement mode outputs.
In accordance with yet a further feature of the invention, the power division signal is formed proportionally to a linear combination of the powers of the measurement modes. Preferably, as mentioned above, the linear combination is a difference between the powers of the measurement modes.
In accordance with a concomitant feature of the invention, a dispersion compensation signal for controlling a coupler phase angle is fed to the coupling element.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an optical filter and method for optical filtering, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.